This invention relates to the manipulation of particles and, more particularly, to the use of lasers to manipulate and sort particles, such as biological cells.
The physical manipulation of single cells selected on the basis of their individual properties plays an important role in experimental cell biology and immunology, as well as in other areas of biomedical research. Two techniques, flow sorting and micromanipulation, have been used in a wide, albeit mutually exclusive, way for the purposes of cell separation or positioning. Flow sorting allows fast, automated, and essentially nonmechanical separation of cells with given optical and electrical phenotypes. However, both electrostatic and fluidic switching flow sorters divert a volume of fluid containing the cell of interest, rather than the cell itself, and thus, have limited positional accuracy. Micromanipulators can position selected cells with micron accuracy, but employ mechanical devices requiring relatively large open volumes for unhindered operation.
Light trapping has also been used to tray and manipulate viruses and bacteria. See, e.g., A. Ashkin et al., "Optical Trapping and Manipulation of Viruses and Bacteria," Science 235, 1517-1520 (March 1987), hereinafter referred to as the "Ashkin reference," and incorporated herein by reference. Optical trapping is based on the transfer of momentum between microscopic particles and the photons they scatter. While the forces produced by this interaction are extremely small, so are the other forces, such as weight and viscous drag, which act on a microscopic particle suspended in a stationary or slowly moving fluid.
Under suitable conditions, the interaction between a collimated laser beam and a microscopic particle results in a radial force proportional to the gradient of beam intensity and in the direction of the intensity gradient, and in an axial force proportional to beam intensity and directed along the beam axis. The radial force can act as a restoring force that traps the microscopic particle on the beam axis. In a strongly focused laser beam, as taught by the Ashkin reference, the axial force may change sign at a point close to the beam waist, and a full three-dimensional trap results. Short focal length optics are provided and the trap is limited to a small volume close to the focus point. Thus, the particles are trapped about the focus and cannot move along the beam axis for transport.
The limitations in the prior art are addressed by the present invention and an improved laser system is provided for identifying and sorting biological particles.
Accordingly, it is an object of the present invention to accurately position biological particles without mechanical contact with the particles.
It is another object of the invention to optically identify and sort a stream of biological particles.
It is yet another object of the invention to sort biological particles with minimum effect on the particles.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.